(Exposure Light Source)
With the miniaturization and higher integration of semiconductor integrated circuits, improved resolution has been desired of semiconductor exposure apparatuses. This entails a reduction in the wavelength of light emitted from the exposure light source, and a gas laser device has been replacing a conventional mercury lamp as the exposure light source. The exposure gas laser devices currently in use include a KrF excimer laser device which emits deep ultraviolet light of 248 nm in wavelength, and an ArF excimer laser device which emits vacuum ultraviolet light of 193 nm in wavelength. As a next-generation exposure technology, the application of immersion technology to ArF excimer laser exposure is under review. The immersion technology is to fill the gap between the exposure lens and the wafer with a liquid to modify the refractive index, thereby reducing the apparent wavelength of the exposure light source. The ArF excimer laser immersion provides a wavelength of 134 nm when the immersion liquid is pure water. As an exposure light source of a more advanced generation, F2 (molecular fluorine) laser immersion exposure using an F2 laser device may be able to be employed which emits vacuum ultraviolet light of 157 nm in wavelength. The F2 laser immersion is to provide a wavelength of 115 nm.
(Exposure Optical Element and Chromatic Aberration)
Semiconductor exposure apparatuses often use a projection optical system. In the projection optical system, lenses and other optical elements having different refractive indices are combined for chromatic aberration correction. The current exposure light sources have laser wavelengths in the wavelength (ultraviolet) range of 248 nm to 157 nm, within which range there are no other optical materials than synthetic quartz and calcium fluoride that are usable as the lens material of the projection optical system. A monochromatic lens of total refraction type, made of only synthetic quartz, is used as the projection lens for KrF excimer laser. A partially achromatic lens of total refraction type, made of synthetic quartz and calcium fluoride, is used as the projection lens for ArF excimer laser. Since KrF excimer laser and ArF excimer laser have a natural oscillation spectral line width as wide as approximately 350 to 400 pm, the use of such projection lenses produces chromatic aberration with a drop in resolution. The laser light emitted from such gas laser devices therefore needs to be reduced in spectral line width so that the chromatic aberration is negligible. For that purpose, such gas laser devices include a line narrowing module having a line narrowing element (such as etalon and grating) in their laser resonator so as to narrow the spectral line width.
(Immersion Lithography and Polarized Illumination)
The foregoing ArF excimer laser immersion lithography with a H2O medium provides a refractive index of 1.44. This in principle can increase the numerical aperture NA of the lens, which is proportional to the refractive index, by 1.44 times as compared to the conventional numerical aperture. The higher NA, the greater the effect of the polarization of the light source, i.e., the laser light. NA has no effect with TE polarization where the direction of polarization is parallel to that of the mask pattern. With TM polarization orthogonal thereto, higher NA lowers the image contrast. The reason is that the electric field vector at the focus on the wafer is in a different direction in the latter case. The intensity decreases with the increasing angle of incidence on the wafer as compared to when the electric field vector is in the same direction. The effect increases as NA approaches or exceeds 1.0, which case corresponds to the ArF excimer laser immersion. As described above, the illumination system of the exposure apparatus needs to be controlled to a desired polarization state. For such a polarized illumination control, the laser needs to be input to the illumination system of the exposure apparatus as linearly polarized in a desired axial direction.
Description of the laser polarization in an exposure apparatus appears in Denis G. Flagello, Steve Hansen, Bernd Geh, Michael Totzeck, “Challenges with Hyper-NA (NA>1.0) Polarized Light Lithography for Sub λ/4 Resolution,” Proceedings of SPIE, Vol. 5754 (2005), pp. 53-68.
In general, the polarization state of polarized light (i.e., linear polarization, elliptical polarization, or circular polarization) is expressed as the sum of mutually orthogonal polarization components. Unpolarized light has mutually orthogonal polarization components of equal light intensities.
As employed herein, a parameter that indicates the polarization state of laser light the exposure apparatus needs will be newly defined. The ratio of the polarization component in a desired axial direction to all the laser light shall be defined as the degree of linear polarization (LP).
The degree of linear polarization indicates the ratio of the light intensity of the linearly polarized component measured in the desired axial direction to the total energy of the laser light. The laser needs to be polarized so as to maintain the degree of linear polarization at a high level.
The degree of linear polarization LP of laser is measured by the following method. As shown in FIG. 28, a polarizer (Rochon prism) is rotated about the optical axis to measure the transmitted light for a maximum intensity Imax and a minimum intensity Imin. The degree of linear polarization is given by the following equation:LP=(Imax−Imin)/(Imax+Imin)  (1)
Assuming that Imax is the component in the predetermined axial direction where the installation angle γ of the Rochon prism is γ=0°, the installation angle of the Rochon prism for Imin is γ+90°.
In the following description, the polarization state shall refer to linear polarization, circular polarization, or elliptical polarization. The degree of linear polarization shall refer to the parameter expressed by equation (1).
(Conventional Technologies for Increasing the Degree of Linear Polarization)
The conventional technologies for increasing the degree of linear polarization of laser light include ones described in U.S. Patent Application Publication No. 2003/219056 and JP-A-2006-73921.
U.S. Patent Application Publication No. 2003/219056 describes a method in which an optical element intended for laser is arranged so that the optical axis of the laser light runs perpendicularly through the (100) plane of its calcium fluoride crystal. Such an arrangement prevents the degree of linear polarization from deteriorating due to intrinsic birefringence when the light passes through the optical element.
The foregoing conventional technology, however, has the following problem.
When the laser light passes through an optical element in the laser device, the laser light deteriorates in the degree of linear polarization due to the birefringence of the optical element. The birefringence includes stress birefringence which is caused by external mechanical stress and thermal stress, and intrinsic birefringence which intrinsically occurs from the crystal structure without such stresses.
According to the technology described in U.S. Patent Application Publication No. 2003/219056, the optical element is arranged so that the laser light passes perpendicularly through the (100) plane, whereby the degree of linear polarization is prevented from deteriorating due to intrinsic birefringence. The stress birefringence occurring from the application of stresses, however, is highest in the <100> direction perpendicular to the (100) plane. There has thus been a problem that when such an optical element is used as a chamber window, stress birefringence can occur due to the stress for holding the window, the several-atmosphere pressure of the gas in the chamber, thermal stress from laser irradiation, etc.
That is, calcium fluoride crystal windows mounted on a conventional laser gas discharge chamber have been proposed mainly to solve the problem of intrinsic birefringence, whereas the calcium fluoride crystal windows entail the problem of birefringence due to mechanical and thermal stresses.
The crystal is cut at an angle of 17.58° or 26.76° with respect to the (111) plane. The use of such a cut surface on both sides of the chamber windows has had the following two problems. A first problem is that the cut surfaces are not capable of high precision polishing with low surface roughness. This lowers the threshold of surface damage from laser irradiation. A second problem is that when such a crystal is used as a chamber window, the gas pressure of approximately 4000 hPa can cause a breakage in the (111) plane which is prone to cleavage. When the cut surfaces are cut at 17.58° to the (111) plane, the angle formed between the chamber window and the optical axis is 70°, and the Fresnel reflections of P-polarized light and S-polarized light are 4.2% and 30.0%, respectively. While the transmission through the window selects the P-polarized component, the high Fresnel reflection of the S-polarized light makes it difficult to secure the laser output.
In the laser resonator, the laser light reciprocates a number of times through the gas discharge chamber which is equipped with two windows. The P-polarization Fresnel reflection as low as 4.2% can thus cause the problem of low laser output.
JP-A-2006-73921 discloses an optical element for ultraviolet gas laser, such as a window, which is made of a calcium fluoride crystal having two flat surfaces, an ultraviolet ray entering the crystal from one of the flat surfaces and emerging from the other flat surface. At least either one of the flat surfaces is parallel to the (110) crystal plane of the calcium fluoride crystal. With such technology, the degree of linear polarization is prevented from deteriorating due to intrinsic birefringence and stress birefringence. The smooth cut surfaces prevent the production of cracking and defects by laser irradiation.
According to the technology described in JP-A-2006-73921, the degree of linear polarization is prevented from deteriorating due to intrinsic birefringence and stress birefringence, and the calcium fluoride crystal is cut along the (110) plane. Such a configuration, however, has had the possibility of causing cleavage during use because of the chamber gas pressure acting perpendicularly on the window and the mechanical stress for holding the window. That is, there has been the possibility that a slip occurs inside the crystal along the (111) plane or cleavage plane, possibly even breaking the window.